| Gulf is
linked to nutrient declines inside the harbour. Climatic
events like the El Niño-La Niña oscillations can
determine whether nutrients feed onshore. During a La
Niña event in 1999, persistent southeasterly winds
caused a ten-fold reduction in nitrates due to onshore
winds stopping offshore upwelling of nutrient rich
water.
Mahurangi Harbour,
however, appears to behave quite differently.
Productivity patterns there appear to fluctuate more,
especially after heavy rainfall. We think nutrient
levels build up on the surrounding farmland, and are
discharged to the catchment only after the soil moisture
levels reach saturation point after persistent rain.
Alternatively, rainfall discharge may flow over the
denser saltwater and, through a process of "advection",
suck in nutrient-rich water from outside the harbour.
Kaipara Harbour, on the
west coast, is different again. The dominant driving
force there is probably a combination of high offshore
productivity, abundant nutrient-rich, land-based runoff,
and the sheer volume of water flowing in and out of this
harbour over shallow, wind-swept mud and sand flats. The
Kaipara has, at times, almost double the nitrate levels
of Mahurangi Harbour.
Understanding these
processes will help farmers to stock their farms
appropriately, as well as addressing any sustainability
issues concerning overstocking of growing areas.
However, long-term data sets are invaluable as it’s no
good developing sustainability models based only on data
from the good years, leaving farmers unable to sell
oysters in the lean years. In the future, we hope to be
able to predict just how many oysters to put on the
farms for predicted climate scenarios.
We are indebted to
Biomarine Ltd and Pacific Marine Farms Ltd for
collecting data for these studies.
Sean
Handley & John Zeldis
s.handley@niwa.co.nz |
